This invention pertains to a visual service that electively transmits a frame of digital information over a television system.
The art has provided a number of schemes for displaying either a few lines or a page of printed text over a television system. Almost invariably these utilize one or a few horizontal scan lines within the vertical blanking interval for transmitting digital information, and by memory storage then accumulate enough information to ultimately display a full page. Certain systems utilize telephone audio channels, and have a low bit rate.
An overview of such developments is given in the paper by Joseph Roizen, "Teletext in the USA", SMPTE Journal, Vol 90, No. 7, July 1981, pgs. 602-610.
A similar study by Clarke & Fenn, "The UK Prestel Service, Technical Developments between March 1980 and March 1981", Proceedings of Videotex '81, International Conference and Exhibition, May 20-22, Toronto, Canada, Page 147, gives the status of this telephone line bandwidth system in Great Britain.
U.S. Pat. No. 3,891,792, H. Kimura, "Television Character Crawl Display Method and Apparatus", June 24, 1975, gives full details on a one line within the vertical blanking interval mode of transmission that produces one line of moving text on a regular television picture system.
Kaiser & Buehlmair, of Institut fuer Nachrichtenuebertragung, Stuttgart, W. Germany, "Cabletext, Text Distribution on CATV Networks, Symposium Record, 12th International Television Symposium and Technical Exhibition, Montreux, 30 May 1981, CATV Sessions Vol., Page 3." propose a system in which digital information is transmitted continuously on unused channel(s) of a cable TV system, by vestigial sideband radio frequency. No effort is made to utilize a video channel with synchronizing pulses.
Feldman, in "Digital Audio Using Your VCR", Radio-Electronics (magazine, USA), August 1981, forms 14 bit words for left and right stereophonic channels and alternates these along television format lines. His method does not provide reliable operation with data characters because he utilizes a relatively high bit rate and relies upon arithmetic averaging to replace missing bits.